Repository For Disposal Of High-level Radioactive Waste Research Articles

Effects of temperature and confining pressure on the permeability of Beishan granite from high-level radioactive waste disposal repository

Understanding the variation of rock permeability with temperature and confining pressure is the foundation for projects such as high-level radioactive waste disposal, deep geothermal exploitation, and carbon dioxide storage. This paper presents an experimental study of the permeability evolution of thermally damaged Beishan granite under effective stress. The results show that temperature has a significant impact on the permeability of granite, and there is an obvious threshold temperature (400–500 °C), initial permeability and permeability change rate increased gradually with temperature. The permeability of Beishan granite shows non-linear decreasing trend with the increase of effective stress, ultimately tending towards stability. The microstructure of granite after thermal damage can be observed by scanning electron microscopy and used to assist in explaining the microscopic mechanism of permeability changes. A new granite permeability prediction model is proposed, which integrates permeability with temperature and effective stress, providing an important reference for the safety of high-level radioactive waste disposal.

Coupled thermal-hydro model tests of double-layered buffer for nuclear waste repository

In the high-level radioactive waste disposal repository, bentonite is a preferred buffer material. As the buffer material, the hydraulic conductivity must be less than 10−13 m/s and the thermal conductivity should be higher than 0.8 W/(m·K) recommended by International Atomic Energy Agency (IAEA). Due to the decay heat of radionuclide and low thermal conductivity of bentonite, the released heat cannot be quickly transferred to the host rock through bentonite buffer layer and the repository may be in a high temperature state (>100 °C), which compromises both the hydro-mechanical performance of the bentonite and the anti-corrosion behavior of nuclear waste canister, therefore the barrier function of the buffer layer is severely damaged. Thus, a double-layered buffer concept which is based on the dual requirements of thermal conductivity and permeability was proposed for the nuclear waste disposal repository. In this paper, coupled thermal-hydro model tests of double-layered buffer with graphite-bentonite mixture layer and bentonite layer (the layer thickness ratio of graphite-bentonite to bentonite was 2:3) were carried out to investigate the temperature and relative humidity distribution laws and determine whether the physicochemical properties of double-layered buffer change after experiencing the coupled thermal-hydro process. In the model test, a heater simulating the decay heat generated by HLW was placed below the graphite-bentonite layer, and a hydration system simulating groundwater infiltration was installed above the bentonite layer, thus, the heat was transferred from the graphite-bentonite layer to the bentonite layer while water was permeated from the bentonite layer to the graphite-bentonite layer. The temperature and relative humidity distribution were synchronously measured by temperature-humidity sensors. After the model tests, the heater was switched off, and the tested bentonite/graphite-bentonite mixture samples were taken out for subsequent water content, scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. The main conclusions from the test results are as follows. The distributions of temperature and relative humidity in the buffers were mainly governed by the competition between the thermal effect and the water permeation effect, the results of water content at different distances also indirectly verify this conclusion. And SEM results showed that macropores of the bentonite close to the water inlet side were significantly reduced and the pore distribution tended to be uniform along the water injection direction during wetting. XRD results indicated that the mineral composition of the graphite-bentonite mixtures close to the heater changed while that of the bentonite away from the heater remained unchanged. Obviously, even if the high thermal conductivity graphite was added to the buffer layer near the heat source, the mineral composition of the bentonite at the heat source side cannot be avoided completely unaffected. Therefore, it is necessary to conduct heat from the inner layer to outer layer to reduce the overall temperature in the repository and reduce adverse effects of temperature and water synchronization on outer bentonite layer. The research results in this paper can serve as a guideline for the buffer layer design in the high-level radioactive waste repository.

Experimental Determination of Polycrystalline Salt Rock Thermal Conductivity, Diffusivity and Specific Heat From 20 to 240°C

Salt rock is recognized as one of the most suitable parent rocks for geological disposal of high level radioactive waste due to its low permeability, good ductility, good thermal conductivity and damage self-healing properties. The thermal conductivity of salt rock directly affects the temperature of disposal storage and surrounding rock, high temperature will lead to a series of problems such as nuclear waste storage tank rupture, mechanical and permeability reduction of surrounding rock. In this paper, the thermal conductivity, specific heat and thermal diffusion coefficient of NaCl single crystal made by crystallization method in the laboratory and polycrystalline salt rock from Khewra salt mine were measured in the range of 22–240°C by the transient plane source method. The results showed that the thermal conductivity and thermal diffusivity of salt rock decrease gradually with the increase of temperature, while the specific heat capacity increases with the increase of temperature. The thermal conductivity of salt rock is slightly lower than that of single-crystal NaCl. The reason for this phenomenon may be that there are a few pores in salt rock. Based on the experimental data, the models of thermal conductivity, thermal diffusivity and specific heat of salt rock with temperature were established. The results can provide a reference for the construction of underground salt rock high-level radioactive waste disposal repository. Based on the thermal conductivity model of polycrystalline salt rock established in this paper, the temperature field evolution during the operation of the underground salt rock high-level nuclear waste repository in 1,000 years was studied. It was found that the temperature of the glass solidified of high-level radioactive waste reached the highest (177.6°C) and then dropped rapidly. The decay heat radiation influence radius of the nuclear waste reaches its maximum in about 50 years of operation of the repository and then gradually decreases.

An optimized potential field geological modeling method for a high-level radioactive-waste geological disposal repository

A three-dimensional geological model that incorporates spatial information efficiently and intuitively is an important basis for the planning, construction, and utilization of high-level radioactive waste (HLW) disposal geological repository sites. However, traditional modeling methods cannot describe extremely complex geological contexts accurately due to the sparsity of effective data. This paper proposed a cokriging gradient interpolation method based on integrated potential fields that combines relationship and orientation data from outcrop descriptions, contour lines, boreholes, and stratigraphic contacts. A multi-stratigraphic sequence was constructed via a curve/surface tracking algorithm, and optimal geological interfaces were screened using universal cokriging equations. The geological modeling for a HLW disposal repository shows that under different data completeness conditions, this method can reveal the spatial distribution of geological structures (including fault networks and intrusive bodies), achieve better approximations for complex geological environments, and provide strong support for HLW repository roadway planning.

Experimental study on gas permeability of granite close to natural joint

The metal corrosion, organic degradation and waste radiolysis in the geological disposal repository of high-level radioactive waste (HLW) will lead to gas generation and form additional pressure, which will drive the nuclide migration and thereby influence the long-term safety of geological disposal. It is crucial to study the gas permeability of the host rock for the geological disposal of HLW. Therefore, the gas permeability of some granite samples close to a natural joint in the Beishan pre-selected area for China’s geological disposal project was studied in this paper. Firstly, the effective gas permeability under different confining pressures, different gas pressures and different directions was measured by the quasi-steady method. Secondly, the absolute gas permeability and slip factor were obtained according to Klinkenberg’s law. Finally, the effect of confining pressure, direction and distance to the joint surface on gas permeability were analyzed. The gas seepage in granite is found to conform to Klinkenberg’s law. The absolute gas permeability is gradually decreased with the increase of the confining pressure. The gas permeability of granite is isotropic and homogeneous because the intrinsic permeability measured in different directions and at different distances from the joint is found to be similar. The above results can provide references for the performance assessment of the surrounding rock of the HLW disposal repository.

Stability Analysis of Host Claystone under T-M Coupling of HLW Disposal Repository in China: Experiments and Numerical Simulation

This paper focuses on the stability of the high-level radioactive waste (HLW) in the proposed clay rock in Tamusu area of China. The in-situ stress as well as the variational characteristics of ambient temperature caused by nuclide decay during HLW storage should be noticeable. A series of thermal property tests and thermo-mechanical coupled strength (T-M) tests of rock samples in the target formation are carried out. Then the stability of surrounding rock of an HLW under the combination of heat release from HLW and in-situ stress is simulated and analyzed by numerical method. Thermal properties of Tamusu clay rock samples are obtained by testing their thermal conductivity. In order to obtain the characteristics and the failure modes of rock samples at different temperatures, the T-M coupling experiments in the temperature range of 100°C are conducted. Numerical model for simulating the state of operation of the nuclear waste tank buried in the tunnel within 100 years is constructed. A thermal boundary by the heat release equation of HLW and the real in-situ stress level in Tamusu area are considered in the model. While, the variation law of surrounding rock’s temperature, stress, and deformation corresponding to the embedding time is obtained from the numerical calculation. Finally, the stability of the deep geological repository is comprehensively evaluated. The results show that the temperature has a significant impact on the T-M coupling characteristics of Tamusu clay rock, and the proposed repository numerical model has no large deformation and failure problems in 100 years. However, the temperature of the surrounding rock of the repository may exceed the safety standard value during the operation period.

Influence of in-situ stress on rock fragmentation by TBM cutters and prediction of TBM performance for deep tunnels

In-situ stress is a significant factor influencing TBM tunneling for deep tunnels, especially for the construction of deep repository for radioactive waste disposal. Beishan area of Gansu Province is the first preferred area for high-level radioactive waste disposal repository in China, and the feasibility of using TBM technology in Beishan area is investigated in recent years. In this paper, six levels of in-situ stress related to different depths from ground to 500m at Beishan area were selected and the influence of in-situ stress on rock fragmentation was investigated by linear cutting test. The rock samples were collected from the Jijicao site in Beishan area, with uniaxial compressive strength of 105.6MPa. The test results show that: 1) there is a clear power function correlation between the penetration and the normal cutter force for each stressed condition, and the higher the confining stress, the smaller the gradient of the power function curve, which means slower increase of penetration rate under the same cutterhead thrust; 2) the required normal cutter force increases with the increase of overburden as the corresponding in-situ stress increases. Based on the laboratory testing, the Specific Rock Mass Boreability Index model was modified and further used in the prediction of TBM performance at different depths on Jijicao site. This paper helps better understanding of rock fragmentation mechanism in TBM tunneling in stressed ground, and also provides reference on parameter selection for TBM operation in deep tunnel tunnelling.

Granular Bentonite Preparation and Effect of Granulation Behavior on Hydromechanical Properties of Bentonite

Compacted bentonite is regarded as a suitable buffer/backfill material in a high‐level radioactive waste disposal repository due to its low permeability, high swelling, and strong adsorption capacity, but the bentonite powder is difficult to compact to a high density only by increasing the compaction energy. Hence, a proposal is made to improve the compactness of bentonite by changing its grain size distribution. To obtain bentonite granules with different sizes, a granulation method is proposed in this paper, in which bentonite powders experience the processes of wetting and drying into plate bentonites, which are then crushed into granules. Furthermore, in this paper, the hydromechanical behavior of granular bentonite is evaluated to verify the feasibility of the method for preparing granules. For this purpose, the granular and original bentonite powder with similar grain sizes were prepared; then, various laboratory tests, including measurements of free swelling ratio, swelling pressure, water retention capacity, and compactness, were carried out. The test results show that the free swelling ratio, swelling pressure, permeability coefficient, and water retention capacity of granular bentonite are almost similar to those of original bentonite, and after compaction, the maximum dry densities of granular and original bentonites are 1.72 and 1.64 g/cm3 at the optimum moisture content (20%) and the energy consumption was reduced by 38% with the void ratio decreased from 1.30 to 0.8 compared with the original bentonite. It indicates that, compared with the original bentonite, the hydromechanical behavior of granular bentonites changed a little, but its compaction performance has been significantly improved. To investigate the differences in pore size distributions of granular bentonite and original bentonite, MIP and NA tests were performed on samples produced with the wetting‐drying agglomerate method, and the results show that the pores with a size of 10.0 μm almost disappear and the pores mainly exist with a size of approximately 1.0 μm. It can be verified that preparing granules by the wetting‐drying agglomeration method is feasible, the granulation process has little effect on the hydromechanical properties of bentonite, and after granulation, not only the compactness is improved but also the energy consumption is saved.

Rock mass characteristic of Suanjingzi section in the Beishan Preselected Site of China’s high-level radioactive waste disposal

Rock mass characteristic is a key influence factor for site selection and site suitability evaluation of high-level radioactive waste disposal engineering. In this paper, rock mass characteristic including geological characteristic, geometric features of joint, and quality evaluation of rock mass was studied for the Suanjingzi section in the Beishan preselected site of China’s high-level radioactive waste (HLW) disposal. Site investigation, mathematical statistics, and probability analysis were applied to study geometric features of joint. The statistics results showed that there are four optimal joint sets in the Suanjingzi section, the occurrence of each joint set obeys Fisher distribution, and the spacing of each set is fitted by negative exponential distribution. Tangent circular sampling window was used to study the trace length and the trace midpoint density, and wide spacing, long trace length, and small trace midpoint density are the main characteristic of Suanjingzi section. Considering the importance of joint in high-level radioactive repository, joint structure rating (JSR) was used to evaluate the quality of rock mass. The results indicated that the Suanjingzi area in the Beishan preselected site was suitable for China’s HLW disposal repository.

Mobilized Mohr-Coulomb and Hoek-Brown Strength Parameters during Failure of Granite in Alxa Area in China for High-Level Radioactive Waste Disposal

Strength parameters of the host rock is of paramount importance for modelling the behaviors of underground disposal repository of high-level radioactive waste (HLW). Mobilization of strength parameters should be studied for a better understanding and modelling on the mechanical behaviors of the surrounding rock, considering the effect of temperature induced by the nuclear waste. The granite samples cored from NRG01 borehole in Alxa candidate area in China for HLW disposal are treated by different temperatures (T = 20 °C, 100 °C and 200 °C), and then are used to carry out a series of uniaxial and tri-axial compression experiments under various confining pressures (σ3 = 0, 5, 10, 20, and 30 MPa) in this study. With the recorded axial stress—axial strain and axial stress—lateral strain curves, mobilization of both Mohr-Coulomb and Hoek-Brown strength parameters are analyzed with the increasing plastic shear strain. It has been found that NRG01 granite samples show generally similar cohesion weakening and friction strengthening behaviors, as well as the non-simultaneous mobilization of Hoek-Brown strength parameters (mb and s), under the effect of various treatment temperatures. Furthermore, the samples treated by higher temperatures show lower initial values of cohesion, but their initial friction angle and mb values are relatively higher. This should be mainly owing to the thermally induced cracks in the samples. This study should be helpful for a better modelling on the mechanical behaviors of NRG01 granite samples as the host rock of a possible HLW disposal repository.

Rock Mass Characteristics in Beishan, A Preselected Area for China's High‐Level Radioactive Waste Disposal

Geological and hydrological characteristics, joint geometric features, rock physical and mechanical properties and rock mass quality are studied in the Beishan area, preselected for China's high‐level radioactive waste (HLW) disposal engineering. A comprehensive survey method is developed to study joint geometric features in the outcrop and samples from borehole BS06 into the Xinchang rock mass were tested. The optimal joint sets are determined by rose diagrams and equal‐area lower hemisphere plots of joint poles. Results show that: 1) the distribution of joint occurrence obeys a normal distribution, while the distribution of joint spacing obeys a negative exponential distribution; 2) concentric circular and tangent circular sampling windows are applied to study the trace length and the trace midpoint density. Results indicate that tangent circular sampling window is more stable and reasonable; 3) Beishan granite shows high density, low porosity and high strength based on many laboratory tests and the physical properties and mechanical properties are closely related; and 4) a synthesis index, Joint Structure Rating (JSR), is applied to evaluate the quality of rock mass. Through the research results of rock mass characteristics, the Xinchang rock mass in the Beishan preselected area has the favorable conditions for China's HLW disposal repository site.

Site screening of clay formations in NW China as host rocks for a high-level radioactive waste disposal repository

Abstract Deep geological disposal of high-level radioactive waste (HLW) in a repository with a system of engineered and natural barriers has been recognized as an appropriate disposal concept by Chinese authorities since 2003, and both crystalline rocks and argillaceous rocks are considered as the candidate host rocks for HLW disposal repository. The 1:200 000 regional survey indicated that there are potential clay formations in Mesozoic–Cenozoic sedimentary basins in NW China. Five candidate areas have been suggested with potential clay formations including the Tamusu and Suhongtu areas with upper K 1 Bayingebi clay formations in the east Bayingebi Basin, in the Inner Mongolia Autonomous Region. On the basis of a detailed ground geological, hydrological and geophysical surveys, two test boreholes drilled to a depth of 800 m in the Tamusu area revealed that there are three lacustrine-facies clay formations (K 1 b 2-3 , K 1 b 2-2 and K 1 b 2-1 ). The thickness of the K 1 b 2-3 and K 1 b 2-2 clay formations is about 300–600 m with sandstone and siltstone interbeds, while the thickness of the K 1 b 2-1 homogeneous clay formation is more than 200 m with the depth of 450 m below the surface. The spatial extension of the clay formations could meet the fundamental criteria to ensure the long-term safety of the repository. Initial mineralogical studies on core samples indicated that the mineral assemblage is dominated by analcite, kaolinite, illite and dolomite. The homogeneous argillaceous rocks rich in analcite in Tamusu area could be a new type of host rock for a HLW disposal repository.

Investigation and Study on Environmental Radioactive Background in Xinchang and Jijicao Rock of Beishan Preselected Region for HLW Disposal Repository in China

On the basis of field environmental investigation and monitoring, the environmental radioactivity background of Xinchang and Jijicao rock in Beishan preselected region has been preliminary investigated and studied, and the public dose from local natural background radiation is estimated which can provide basic data and information for environmental impact assessment and safety assessment of HLW(the high level radioactive waste) disposal repository in the future. From the result of investigation and study, the environmental radioactivity of Xinchang and Jijicao rock is generally within normal natural background. The effective dose to local resident from natural background radiation is 2.110 mSv/a by internal and external exposure.

Study on Modelling in Biosphere for Performance Assessment on HLW Disposal Repository in China

Based on BIOMASS methodology, FEPs analysis and data preparation, the biosphere model for Beishan, China (BIOMOBEIC) for performance assessment on high level radioactive waste(HLW) disposal repository has been developed by utilizing AMBER which is an efficient compartment modeling tool in order to evaluate dose rate to individual due to long-term release of nuclides from the HLW repository. From the result of mathematical simulation, the biosphere dose conversion factors (BDCFs) are obtained which are critical factors for conversion of release rates from the geosphere to individual doses in biosphere assessment and performance assessment.

Application of the tri-axial drill-bit VSP method to drilling for geological survey in civil engineering

We have examined the applicability of the triaxial drill-bit VSP method (TAD-VSP) to the geological survey of possible sites for a high-level radioactive waste disposal repository. The seismic energy generated by a drill bit is measured by a downhole multi-component detector, and the resulting signals are processed to image the geological structure deep underground. In order to apply the TAD-VSP method to civil-engineering-scale drilling, we have developed a small but highly sensitive and precise three-component downhole seismic measurement system, and recorded drill-bit signals at a granite quarry. We have successfully imaged discontinuities in the granite, possibly related to fractures, as highly reflective zones. The discontinuities imaged by the TAD-VSP method correlate well with the results of other borehole observations. In conclusion, the TAD-VSP method is usable in geological investigations for civil engineering because the equipment is compact and it is simple to acquire the drill-bit signal.

Detecting Strain in the Yucca Mountain Area, Nevada

From repeated surveys of the relative motions between geodetic stations Claim, Black, Mile, 67TJS, and Wahomie (Fig. 1) from 1991 through 1997, Wernicke et al. (1) deduce a N65°W strain accumulation rate of 50 ± 9 nanostrains per year (2) across the proposed high-level radioactive waste disposal repository at Yucca Mountain, Nevada. That strain rate is sufficient to indicate a higher-than-expected earthquake hazard at the repository. An earlier (1983–1993) U.S. Geological Survey (USGS) measurement (Table 1) of the strain accumulation in the network (black triangles) in Fig. 1 found a N65°W strain rate of 8 ± 20 nanostrains per year, a rate not significantly different from zero (3). Although the two measurements are marginally consistent at the 95% confidence level [that is, the difference 42 ± 22 nanostrains per year is less than 2 standard deviations, (SD)], the earthquake hazard implications are somewhat different. The USGS strain determination is the better of the two measurements. Wernicke et al. did not include the effects of monument instability in their error budget and, as a consequence, have significantly underestimated the uncertainties in their measurements. Moreover, in their interpretation of the data, they did not give proper weight to the coseismic and postseismic effects of the Little Skull Mountain earthquake (June 29, 1992, M = 5.4).